HDDS-16385. Minor cleanups in NodeDecommissionManager

[navinko]

What changes were proposed in this pull request?

Fix typo and remove redundant list copy in NodeDecommissionManager.

1. In NodeDecommissionManager.parseHostname(), the InvalidHostStringException message has a typo — "does not contain a value hostname or hostname:port definition" should read "a valid hostname".
2. In NodeDecommissionManager.checkIfMaintenancePossible(), the list copy is redundant
   List<DatanodeDetails> validDns = dns.stream().collect(Collectors.toList());
   Followed by Collections.copy(validDns, dns);
   Which copies dns twice, and should be simplified to List<DatanodeDetails> validDns = new ArrayList<>(dns);
   Which is matching with other implementations like checkIfDecommissionPossible() definition.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-16385

How was this patch tested?

No behaviour changes, Ran existing unit tests in TestNodeDecommissionManager
Successful CI Build : https://github.com/navinko/ozone/actions/runs/33965658546